The present invention relates to fuel nozzles for gas turbines. More specifically, the present invention relates to a fuel nozzle assembly, along with a method of making and repairing same, suitable for use with liquid or gaseous fuels and having the capability for steam injection for NOx control.
Gas turbines include one or more combustors adapted to produce a hot gas by burning a fuel in compressed air. A fuel nozzle assembly is employed to introduce the fuel into the combustor. To provide maximum flexibility to the user, such fuel nozzles are often of the dual fuel type--that is, they have the capability of burning either a liquid or a gaseous fuel, or both simultaneously.
Unfortunately, combustion in gas turbine combustors results in the formation of oxides of nitrogen (NOx) in the combusted gas that is considered an environmental pollutant. One method of minimizing the formation of NOx involves injecting steam, via the fuel nozzle, into the combustor along with the fuel. However, such steam injection can not be readily accomplished in a traditional dual fuel nozzle so that the flexibility of a dual fuel nozzle is lost. Another problem with steam injection via conventional fuel nozzles is that it is difficult to introduce the steam symmetrically around the longitudinal center line of the combustor so as to prevent non-uniformities in the combustion gas.
A further drawback of traditional fuel nozzles is that they are subject to deterioration of the nozzle cap due burning and erosion from exposure to the hot combustion gases. If oil fuel is being burned, such nozzles are also subject to coking at the fuel outlet port. Consequently, replacement of the nozzle is a frequent occurrence. This presents a maintenance problem for the user.
It is therefore desirable to provide a fuel nozzle for a gas turbine capable of burning gaseous or liquid fuel, or both simultaneously, along with injecting steam. It would also be desirable to provide a method of replacing the nozzle cap portion of such a fuel nozzle.